A known diesel exhaust after-treatment system comprises a DOC in upstream flow relation to a catalyzed diesel particulate filter (CDPF). A CDPF is a diesel particulate filter (DPF) that has some catalyst on it. While a CDPF is more commonly used for filtering particulate matter, it is known to use a DPF without any catalyst for the same purpose, and the two terms are often used interchangeably. For the after-treatment system to effectively treat exhaust, the temperature of the exhaust must exceed the “light off” temperature of the DOC. In a turbocharged diesel engine, the DOC is in downstream flow relation to the turbocharger turbine, and for certain engine operating conditions, such as light engine load and low engine speed, the temperature of exhaust leaving the turbine may not exceed the DOC “light off” temperature, which is typically around 270° C. Failure of the DOC to “light off” can lead to face plugging by entrained matter in the exhaust, and that can have undesired consequences on operation of the after-treatment system and its components. Attaining “light off” temperature becomes more difficult as the ambient temperature gets colder, and is perhaps most noticeable at engine cold start.
Consequently, it would be desirable to accelerate the attainment of DOC “light off” temperature at engine starting, especially at cold ambient, and to maintain “light off” temperature over a more extensive range of engine operation, especially operation at light loads and low speeds.